This invention relates to a process for manufacturing phosphorous acid or hypophosphorous acid from a sodium phosphite or sodium hypophosphite, respectively. In particular, it relates to the reaction of a sodium phosphite or sodium hypophosphite with hydrogen chloride to precipitate sodium chloride and form phosphorous acid or hypophosphorous acid, respectively, which can be purified by anion exchange.
Phosphorous acid, H.sub.3 PO.sub.3, has a number of industrial applications including as an intermediate in the manufacture of water treatment sequestrants, and drilling fluid components in selected down-hole oil well applications, and for use in pH adjustments in the sodium hypophosphite process. Phosphorous acid is usually produced by reacting water with phosphorus trichloride, but that is an expensive process due to the high cost of phosphorus trichloride. Phosphorous acid can also be derived as a by-product of reactions that produce alkyl chlorides, but phosphorous acid made by those processes is contaminated with organic materials, which reduces its value or requires downstream organic removal steps.
Hypophosphorous acid, H.sub.3 PO.sub.2, also has a number of industrial applications such as, for example, in making polymer stabilizers and veterinary medicines, and as a reducing agent for the precipitation of metal ions from solution, converting them to elemental metals. Hypophosphorous acid can be made by reacting sulfuric acid with sodium hypophosphite, which produces hypophosphorous acid and sodium sulfate. That process requires refrigeration to a temperature of about -40.degree. C. in order to precipitate the sodium sulfate. The hypophosphorous acid liquor must then be treated with barium to precipitate any residual sulfate ions as barium sulfate. These procedures add considerably to the cost of producing hypophosphorous acid, and result in the need to dispose of barium sulfate "mud" and hydrated forms of sodium sulfate.
Hypophosphorous acid can also be made by passing sodium hypophosphite through a cation exchange column, where sodium is exchanged for hydrogen. This process requires a very large cation exchange column, and also generates an aqueous waste stream that contains phosphorus, which is difficult to dispose of.